Find a good resource with lots of difficult problems to solve. The resource is basically useless if it doesn't come with a well written answer key.

Leetcode is good for learning leetcode. The Art of Problem Solving books are good for pure math. Stuff like that.

That'll get you through the "basics" if you really like math and also go to undergrad/grad school for it. If you want to be "the best" then you'll have to compete with people who actually are geniuses. Idk how to compete with them, maybe read their blogs and try to replicate their practice methods?

Here's a really smart guy's blog: https://terrytao.wordpress.com/